Spirals in Time: The Secret Life and Curious Afterlife of Seashells (26 page)

The Sedgwick Museum of Earth Sciences in Cambridge has display cases filled with coprolites. Many of them were found by Harry Seeley, an assistant to Cambridge’s professor of geology in the mid-nineteenth century, Adam Sedgwick. Throughout the 1860s, Seeley paid regular visits to the nearby coprolite pits where he picked through the washing tanks to see what interesting and unusual specimens were turning up. On display today at the museum are grey and black ammonites, as well as bivalves and gastropods.

Besides the few specimens liberated by Seeley, estimates suggest another two million tonnes of phosphate-rich fossils were dug up and loaded onto horse-drawn carts, steam trains and barges and taken away to be crushed in windmills. Sulphuric acid was then added to the powder to make ‘superphosphate’, which was sold for half the price of Peruvian bird droppings and was exported across the globe. Until cheaper sources of rock phosphate were found in the 1880s and coprolite production fell, arable crops from Russia to Australia were grown with the aid of some very old seashells.

Fossil ammonites have left another, more lasting legacy in the human world. Two hundred years ago, British engineer William Smith was the first person to realise that fossils, and in particular ammonites, were time capsules that declare the age of rocks. His job involved travelling the country, digging a new network of canals. He noticed that as his men dug deeper the rocks changed, and so did the fossils inside them. He gathered together a fine collection of fossils, including many ammonites, and used them to prove that rocks are deposited in flat layers like pancakes; later those flat rocks can become squashed, tilted and folded as the Earth’s crust shifts.

Several features of ammonites made them extremely useful to Smith as he probed geological formations. Not only were their fossils immensely abundant and easy to find, but there were also thousands of ammonite species (many can be identified from intricate patterns like fingerprints, called sutures, etched across their fossilised shells; these were the junctions between the internal chamber walls and outer shell, with the lines revealed when sand and mud filled an empty shell, then formed an internal mould). Individual species also tended to be quite short-lived, appearing and then going extinct in a geological heartbeat, sometimes just a few hundred thousand years. This means that if the same ammonite species is found in different locations, the rocks
they lie in must be roughly the same age. This is the basis of a powerful geological technique known as biostratigraphy. With their cosmopolitan ranges, ammonites assist geologists in linking rock formations on opposite sides of the planet. The same species have been found in Chile, Australia, Europe, Madagascar, China and Antarctica.

By matching the ages and types of rocks in different places, Smith drew an enormous map, two metres (more than six feet) tall, showing in fine detail the geology of England, Wales and part of Scotland. With different colours for different rock formations, he produced a rainbow view of the British Isles that had never been seen before. The map and Smith’s findings played an important part in the emerging science of geology, helping to advance theories of how rocks are formed over millions of years.

You say ammonite, I say ammonoid

A confusing thing about ammonites is that, technically, rather a lot of them should in fact be referred to as something else. The lineage that ammonites belong to – the ammonoids – split from the rest of the cephalopods in the Devonian around 400 million years ago. The true ammonites showed up more than 200 million years later, in the Early Jurassic. Before then, dozens of other ammonoid groups came and went. People usually refer to them all as ammonites, but in fact they were different, closely related animals.

In Palaeozoic seas, from the Devonian onwards, the dominant ammonoids were the goniatites, most of them with small, compact spiralling shells. They thrived until 252 million years ago when a crisis hit the living planet, one like none that had come before. The End-Permian mass extinction, also known as the ‘great dying’, was probably triggered by a combination of colossal volcanic eruptions, the bubbling up of methane from the deep sea and subsequent runaway global warming. It wiped out 70 per cent of life on land and 96 per cent of ocean-going species, including the last of the trilobites.
Even though the goniatites went extinct, the ammonoid lineage survived into the Triassic. The oceans filled with the next major ammonoid group, the ceratites. They were quite short-lived, with a reign that lasted only 50 million years or so. Then one final, grand assembly of ammonoids took centre stage. From the early Jurassic onwards, the oceans were teeming with ammonites.

Even though their fossils are incredibly abundant, the ammonites and their relatives remain deeply mysterious creatures, and many of their secrets remain locked in the past. Apart from their shells, we don’t know what ammonites looked like. So far, not a single fossil ammonite has been found with its soft body preserved. Did they have eight arms like octopuses? Eight arms and two tentacles like squid and cuttlefish? Or did they have dozens of noodly appendages like nautiluses? We don’t know.

One thing we do know is that they probably swam around by jet propulsion. A notch in the opening of ammonite shells hints that they had a fleshy funnel, like living cephalopods. It’s mind-numbing to imagine the biggest known ammonite,
Parapuzosia,
pulsing its way through the seas – fossils of their shells are two metres (six and a half feet) in diameter. Experts think the living creature could have been three metres across, and weighed a tonne and a half or more. If giants drove around in monster trucks, these shells would be their wheels.

There were plenty of other strange sights in the oceans during the reign of the ammonites. On the whole, their shells were sculpted into neat spirals; they occupy just a small corner of David Raup’s museum of all possible shells. Some ammonites, though, did things completely differently.

Helioceras
was an ammonite with a tall, helical shell covered in spikes that looked like a dangerous helter-skelter. They would have hung with their heads down, and a gentle puff of water from their funnel would have sent them into a spin. Perhaps they pirouetted up and down through the seas like
corkscrews.
Nipponites
was another strange ammonite. It had a meandering shell, tangled up in knots, similar in appearance to (but much bigger than) the microsnails that live today in the chalk hills of Borneo.

Something else we don’t know about ammonites is what they ate. Rare fossils have been found with what could be their stomach contents, including little creeping crustaceans called ostracods and flower-like relatives of sea urchins called crinoids, as well as other ammonites. But not everyone agrees that these definitely were the ammonites’ last meals. What is clear, though, is that other animals were eating ammonites. They were not the highest-ranking predators in the oceans, as their Ordovician ancestors were. The hunters had evolved into the hunted.

Fossil ammonites have been found with smooth, round holes in them and some experts think these are scars left by limpets that latched on after the ammonites died. Further analyses, however, point towards a more brutal endgame.

Jurassic ammonites shared the oceans with plenty of scary beasts, including dolphin-like reptiles, the ichthyosaurs, followed later by mosasaurs. These were terrifying marine lizards, up to 20 metres (65 feet) long with huge snapping jaws packed with teeth that just happen to match the size and spacing of the holes in many ammonite shells. Rather than limpet scars, a more likely explanation is that the holes are indeed tooth-marks. There seems to be no obvious reason why limpets would line themselves up, time and again, into the same V-shaped arrangements.

One ammonite has been found with punctures in two sizes: a perfect fit for adult and juvenile mosasaur teeth. Was an adult mosasaur teaching its offspring how to hunt? Or did it sneak up on a youngster and steal its dinner? Either way, it wasn’t good news for the ammonite.

Meanwhile, as giant swimming reptiles were chasing after ammonites, new threats to everything in the oceans were approaching. Soon the reign of the shelled cephalopods
would come to an end, leaving one final, big question: why are there no ammonites around today?

Ammonites well and truly hogged the cephalopod limelight throughout the Mesozoic. Meanwhile, in the background, another group of shelled cephalopods were quietly getting on with things. These were the nautilids. From the outside, they looked a lot like ammonites but compared with their more famous cousins, they lived in much smaller populations and there were not nearly as many species.

Side by side, the ammonites and the nautilids survived multiple mass extinction events, and kept going until 65.5 million years ago. Then, at the end of the Cretaceous, a mass extinction came along that only one of these two groups would survive.

This is probably the most famous mass extinction of all, because on land it saw the end of the non-avian dinosaurs. It also hit the oceans hard: only one in five marine species pulled through into the Tertiary, and I certainly would have put my money on ammonites being among the survivors, rather than nautilids. There were far more of them, and they were more widespread, two factors that normally create a buffer against extinction. Even so, it was the ammonites that bade farewell to the planet while the nautilids persisted, giving rise eventually to the chambered nautiluses. And for a long time, palaeontologists have wondered why.

To pin down the causes of extinction is difficult enough in the present day. Even when biologists can tiptoe up to endangered species, watch them and test out ideas of why they are in trouble, it can still be a great challenge to decipher the real issues (and even harder to do something about them). Imagine, then, how much more difficult it is when the species in question are already long gone, leaving behind only traces of themselves in rocks. All we have are theories. Researchers have scrutinised the ammonites, then the nautilids and details
of the mass extinction, hunting for explanations of what happened and what went wrong for the ammonites.

The longest standing theories about why ammonites lost out are linked to the way they are born. Hatchling ammonites, known as ammonitella, were tiny. We know this because, if you look carefully, you can see the smooth, inner whorls of a fossilised ammonite shell that grew in predictable conditions while it was still inside its egg, feeding off yolk. As soon as it hatched and had to fend for itself in the erratic outside world, new shell layers became irregular. For ammonites, those uneven whorls began to appear when the shell was only one millimetre across. Young nautilids, on the other hand, were around ten times bigger when they hatched. It’s thought that at a tender age, these two groups were doing very different things. Ammonites were drifting through the water, as part of the plankton, while nautilids probably stuck closer to the seabed.

This difference may not have mattered too much when the going was good, but it could have been the downfall of ammonites when things got stressful. The exact cause of this game-changing mass extinction is still hotly debated. The fossil record shows that leading up to it, the great ammonite lineage was already in decline, with many genera going extinct. Falling sea levels, which dropped by as much as 150 metres (500 feet) in one million years, may have had something to do with it.

Then, the infamous asteroid, Chicxulub, slammed into Mexico’s Yucatán Peninsula, casting dust clouds across the Earth and triggering a long, dark winter. Many experts think this alone explains the extinctions, while others argue that massive volcanic activity in India also had its part to play in the downfall of life on Earth. Today, the Deccan Traps in central India consist of a layer of solid basalt, two kilometres deep and half a million square kilometres in area, which gives an idea of just how enormous these volcanic eruptions and lava flows were. They would have spewed
carbon dioxide and sulphur dioxide into the atmosphere, contributing to the planet-wide changes.

Sulphurous gases in the atmosphere would have combined with water and fallen in showers of acid rain; this would have turned shallow seas more acidic and could have made life distinctly uncomfortable for planktonic species, including young ammonites, floating around inside chalky skeletons. By contrast, the next generation of nautilids were tucked up safely down in the deep sea, out of reach of the worst effects of these corrosive waters.

Diet may also have had a part to play in the ammonites’ demise. In 2011,
Isabelle Kruta
and colleagues conducted a detailed three-dimensional scan of an ammonite called
Baculites
. She found what she thinks are remains of its last meal, including the planktonic shell of a gastropod larva. Other experts contend that we can’t be sure if this plankton really was food or just a passer-by that got caught in the same rock. But if ammonites did have a microscopic diet, then a collapse of planktonic populations – triggered by corrosive, warming waters during the extinction event – could have left adult ammonites starving.